(a) Field
An embodiment of the present invention relates to a converter and a driving method thereof. More particularly, the embodiment relates to a switch control circuit that can prevent hard-switching, a converter using the same, and a switch control method.
(b) Description of the Related Art
A zero current detection structure is required to control switching operation of a converter switch forming a power factor correction circuit. Zero current detection implies to detect a time point that a current flowing to an inductor of the converter becomes zero. In the converter, the switch is designed to be turned on when the current flowing to the inductor becomes zero.
A conventional power factor correction converter uses an auxiliary wire coupled to a converter inductor in an insulated manner with a predetermined turn ratio for zero current detection. A converter control circuit includes a separate pin, and is connected to the auxiliary wire and receives a zero current detection voltage corresponding to a voltage at both ends of the inductor. The converter control circuit detects a time point that the inductor current becomes zero using a zero current detection voltage, and turns on the switch at the detected time point.
However, a converter control circuit that does not include a separate pin for zero current detection directly detects a current flowing to an inductor for zero current detection. At a time point that a voltage (hereinafter, a zero detection voltage) detects a current flowing to the inductor becomes zero, a switch is turned on.
According to such a method, the switch is turned on after a predetermined time period is passed from a time point at which the detection voltage reaches a predetermined threshold voltage. A drain voltage of the switch is decreased due to resonance after the switch is turned off, and a resonance frequency is determined by the inductor and a parasite capacitor of the switch.
A delay period is determined to control the switch to be turned on when the drain voltage becomes the lowest voltage according to the resonance frequency. That is, for soft-switching, the switch is designed to be turned on after the delay period is passed from a time point that the detection voltage reaches the threshold voltage.
However, when the inductor and the parasite capacitor are changed, the resonance frequency is fluctuated such that hard-switching may occur. More particularly, when the resonance frequency is increased or decreased, the drain voltage is maintained in high level even though the delay period is passed so that hard-switching occurs.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.